When aluminum is extracted electrolytically from alumina dissolved in a cryolite-based molten bath (Na3AlF6+other stable fluorides), sodium is codeposited with the aluminum. Small proportions of other fluorides are employed, such as lithium fluoride or magnesium fluoride, which can confer advantageous properties on the bath leading to improved economics, resulting in traces of lithium and magnesium in the pool of molten aluminum. The concentrations of these light alkali and alkaline earth metals are dependent upon physical and chemical conditions within the electrolytic reduction cell and the manner in which it is operated. Typical concentrations of sodium, lithium, magnesium, and calcium in freshly tapped molten aluminum metal are in the ranges of from 40 to 180 PPM, 5 to 25 PPM, 5 to 150 PPM, and 4 to 10 PPM respectively and are contaminants which must be removed or lowered to below specifications for the aluminum metal to be marketed for many uses. For many primary aluminum products of casting operations, the specifications require levels of Na and Li to be less than 2 PPM and in some cases as low as 0.5 PPM.
The prior art teaches treatment of primary aluminum metal for the removal of traces of alkali metals by passing through a packed bed of carbonaceous material mixed with solid, granular aluminum fluoride, such as disclosed in U.S. Pat. Nos. 3,305,351 and 3,528,801. Although a bed of alternating layers from 1 to 5 feet deep containing from 5 to 90 wt. % AlF3 can treat from 50,000 to 100,000 pounds of primary aluminum metal, there is difficulty in keeping the bed open, porous, and combusting uniformly, promoting the dispersion of aluminum fluoride. The residue from the spent bed presents environmental problems for disposal because it contains potentially hazardous waste material.
U.S. Pat. No. 4,277,280 discloses flowing molten aluminum through a bed of coarse, granular AlF3-containing material wherein the reactive aluminum fluoride can combine with the alkali metals to form a liquid phase and cause clogging of the initial granular bed. Bath components unavoidably tapped with the liquid aluminum from the cells can also react with the AlF3 to form either unwanted liquid or channels in the bed, thus decreasing its effectiveness.
U.S. Pat. No. 4,470,846 discloses removal of contaminating alkali and alkaline earth metals by reaction with aluminum fluoride-yielding material dispersed into a stable vortex in the molten aluminum surface. A rotating impeller with the pitch of the blades designed to force the liquid metal downwardly creates a vortex in the surface of the metal around the shaft connected to a motor. Controlled quantities of granular AlF3 were added into the vortex from a hopper.
U.S. Pat. No. 4,138,246 discloses a filter bed method for lowering the concentration of sodium in molten aluminum, by gravitationally driven flow through a loosely packed filter bed of granular material, which in part, comprises carbon. It suggests that carbon preferentially absorbed a portion of the sodium.
Another form of bed filter for removing sodium, lithium and calcium was described by Achim and Dubxc3xa9 (Light Metals, 1982, pp. 903-916) wherein molten aluminum containing approximately 100 PPM Na and 30 PPM Li flowed through a filter bed of egg-shaped granules of a mixture of cryolite 3%, alumina 9%, and aluminum fluoride 86%. While this application did remove up to 90% of the light metals, the briquettes forming the bed were consumed and the vessel containing the bed for in-line metal cleaning had to be taken down, cleaned and repacked with AlF3-containing 15-50 mm diameter pellets after processing 1000 to 1500 tons of liquid aluminum.
The Mixal process for treating aluminum in potline crucibles (Archard and Leroy, Light Metals 1990 pp. 765-768) is a prior art method wherein a mixture of gases, chlorine-argon or chlorine-nitrogen is injected into molten aluminum through a spinning nozzle or rotor. After about 11 minutes of treatment the concentrations of sodium and lithium is reduced to 5 and 3 ppm respectively. The products of the reaction are volatile and the effluents are drawn off through a covering lid to a lime injected bag filter capture system. Lack of liquid slag or dross from the reactions depleting the alkali metals is an advantage of this process. However, the risk of recycling chlorides mixed with the fluorides to the reduction cells presents the possibility that untenable corrosion will occur in the electrical connections to the buswork and in the fume capture system of the potrooms.
Primary aluminum is obtained by electrolytic extraction from alumina dissolved in a cryolite-based molten bath (Na3AlF6+other stable fluorides). Sodium is codeposited with the aluminum during this process along with lithium and magnesium fluorides, which can confer advantageous properties when they are used, resulting in not only sodium but also traces of lithium and magnesium being present in the pool of molten aluminum. The concentrations of these light alkali and alkaline earth metals are dependent upon physical and chemical conditions within the electrolytic reduction cell and the manner in which it is operated. Typical concentrations of sodium, lithium, magnesium, and calcium in freshly tapped molten aluminum metal are in the ranges of from 40 to 180 PPM, 5 to 25 PPM, 5 to 150 PPM, and 4 to 10 PPM respectively.
These metal traces are contaminants which must be removed or lowered for many commercial products. Primarily aluminum many products of casting operations include specifications requiring levels of Na and Li to be less than 2 PPM and sometimes as low as 0.5 PPM.
Although fluxing with gas mixtures such as N2xe2x80x94Cl2, N2xe2x80x94COxe2x80x94Cl2, Arxe2x80x94Cl2, and single gases such as argon and nitrogen is routinely practiced in holding furnaces and in launders to remove particles, hydrogen and light metals, this is not a preferred method for decreasing the amounts of Na, Li, Ca, and Mg. These metals form halides, promote the formation of extra dross, react with the refractory lining of the holding/melting furnace and require extended time of fluxing to reduce their concentration in typical virgin primary aluminum to lower levels required in semi-finished aluminum products.
The methods presently practiced to remove or reduce the concentrations of light metals include charcoal filtering in which a tapping crucible of primary aluminum is poured into another crucible containing a bed of burning charcoal and AlF3. The combined effects of the agitation of the metal, reaction with AlF3, oxidation from entrained air, and the propensity for sodium to react with carbon in the presence of fluoride, depress the concentration of Na and Li to about 10 and 5 ppm respectively. In both Alcan and Hycast xe2x80x9cTreatment in Cruciblexe2x80x9d methods, a crucible of virgin molten metal is taken to a station where an impeller is lowered onto the open crucible. In the Alcan method, the rotating impeller creates a vortex into which granular AlF3 is added for a period of 5 to 15 minutes. In the Hycast method granular AlF3 is injected into the metal with an inert carrier gas through a hollow axle and into the metal through a rotating impeller for a period of 5 to 20 minutes. As in the previous methods described, there is enough latent heat for the molten aluminum to remain liquid throughout these treatments. Aluminum fluoride reacts according to the equations:
AlF3+3NaF=Na3AlF6
AlF3+3LiF=Li3AlF6
The slags/drosses are skimmed off the surface of the aluminum metal after the rotor is stopped. The treated aluminum metal is thereafter transported to the casthouse holding furnace.
This novel innovative process for removal of alkali and alkaline earth metals from virgin or primary molten aluminum comprises the apparatus and method for injecting a powdered, vaporizable fluoride carried in a gas stream through a rotating impeller, thus creating bubbles and differential shear forces. This normally is accomplished in the tapping crucible at a specially designed metal cleaning station, advantageously located between the potroom and the cast house. The metal cleaning station, in addition to the necessary controls and ancillary equipment to position and activate the rotor/impeller, regulates the mixture of gases and rate of injection of the fluoride-containing powder. The cleaning station advantageously has two positions at which crucibles of aluminum metal on transports can be treated. Preferably there is also an intermediate position at which the impeller and the shaft connecting to the drive motor can be cleaned of any adhering frozen cryolitic bath. Particulate emissions from the crucible during removal of light metals are captured by means of a lid or cover to the crucible and an exhausting system with a multiclone and baghouse system. The present novel system also provides for the mechanized removal of any solid dross or cryolitic material floating on the surface of the molten aluminum after treatment.